System and method for downlink channel sounding in wireless communications systems

ABSTRACT

In accordance with an embodiment, a method of operating a base station configured to communicate with at least one user device includes transmitting a reference signal to the at least one user device, receiving channel quality information from the at least one user device, and forming a beam based on the channel quality information received from the at least one user device.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 16/265,653, filed on Feb 1, 2019, now U.S. Pat. No.10,855,353, entitled “System and Method for Downlink Channel Sounding inWireless Communications Systems,” which is a continuation of Ser. No.15/650,473 filed on Jul. 14, 2017, now U.S. Pat. No. 10,498,416,entitled “System and Method for Downlink Channel Sounding in WirelessCommunications Systems,” which is a continuation of U.S. Non-Provisionalpatent application Ser. No. 14/874,190 filed on Oct. 2, 2015, now U.S.Pat. No. 9,735,852, entitled “System and Method for Downlink ChannelSounding in Wireless Communications Systems,” which is a continuation ofU.S. Non-Provisional patent Application Ser. No. 12/830,983 filed onJul. 6, 2010, now U.S. Pat. No. 9,184,511, entitled “System and Methodfor Downlink Channel Sounding in Wireless Communications Systems,” whichclaims priority to U.S. Provisional Application No. 61/224,737 filed onJul. 10, 2009, entitled “System and Method for Downlink Channel Soundingin Wireless Communications Systems,” all of which are incorporated byreference herein as if reproduced in their entireties.

TECHNICAL FIELD

The present application relates generally to wireless communications,and more particularly to a system and method for downlink (DL) channelsounding in wireless communications systems.

BACKGROUND

In coordinated multi-point (CoMP) transmission, transmissions frommultiple enhanced Node Bs (eNBs) are made simultaneously to a singleUser Equipment (UE). Coordination of the transmissions made by the eNBsenable the UE to combine the transmissions to improve high data ratecoverage and to increase system throughput in advanced wirelesscommunications systems, such as Long Term Evolution-Advanced (LTE-A).eNBs may also be commonly referred to as base stations, base transceiverstations, controllers, access points, and so forth, while UEs may alsobe commonly referred to as subscribers, subscriber stations, terminals,mobile stations, and so on.

In general, there are two different CoMP approaches: joint processingfrom multiple cells (eNBs) and coordinated scheduling/beamforming(CS/CB). In joint processing, there is an assumption that data isavailable at each transmission point (eNB) in a CoMP cooperating setrepresenting eNBs participating in the CoMP transmission. The data maybe transmitted from more than one eNB at a time with dynamic eNBselection when the data is transmitted from only one eNB at a time. InCS/CB, the data may only be available at a serving eNB and transmissionscheduling is coordinated among eNBs within the CoMP cooperating set.

In order to further help achieve better channel utilization and increaseoverall system performance, channel state/statistics/information about adownlink (DL) channel(s) between an eNB and a UE may be provided by theUE to the eNB. The channel state/statistics/information provided by theUE enables the eNB to adjust its transmitter to more effectively makeuse of the DL channel(s) condition.

In general, there may be two types of channelstate/statistics/information feedback scheme for LTE-A: explicit channelstate/statistics/information feedback and implicit channelstate/statistics/information feedback. In explicit feedback, an eNBdetermines the CoMP transmission processing matrix based on the whole ormajor part of the CoMP channel information, and therefore better CoMPperformance can be obtained at the expense of high feedback overhead.With explicit feedback, more information may be provided to the eNB toafford the eNB greater flexibility in scheduling CoMP transmissions. Ifprecoded DL reference signals are used, a selected CoMP transmissionscheme may be transparent to the UE. However, uplink (UL) feedbackoverhead may be high when instantaneous channel information feedback isrequired.

In implicit feedback, an eNB determines the CoMP transmission processingmatrix based on precoding matrix indication (PMI)/rank indication (RI)recommended by UE. For non-coherent multi-point CoMP transmission, onlydisjoint PMI/RI information (or individual PMI for cells in CoMPcooperation set) is required, while for coherent multi-point CoMPtransmission, joint PMI/RI feedback which contains individual PMI/RIinformation and additional inter-cell spatial information or a singlejoint PMI/RI information is required. Usually joint PMI/RI feedbackdemands more feedback overhead than disjoint PMI/RI feedback. Withimplicit feedback, the UE feedsback channel information based on certaintransmit or receive processing and incurs less feedback overhead.However, this may come at decreased scheduling flexibility.

Therefore, channel state/statistics/information feedback should be ableto provide the eNBs with a high degree of control and flexibility with areasonable UL feedback overhead. Feedback schemes that use nonpure-codebook based precoding to provide the eNBs the flexibility todetermine the CoMP transmission scheme are desirable. Additionally, thefeedback schemes should allow the eNBs to override recommendations fromthe UEs. For example, in a situation where transparent CoMP transmissionwith the assistance of a precoded demodulation pilot.

Additionally, LTE-A is capable of supporting advanced forms of multipleinput, multiple output (MIMO), such as single user MIMO (SU-MIMO) ormulti-user MIMO (MU-MIMO). For example, in SU-MIMO and MU-MIMO mayutilize precoding with more than four (4) transmit antennas. The moreadvanced forms of MIMO may require more accurate tuning of a transmitbeam and/or closed-loop (CL) spatial channel to allow for the support ofa variety of antenna configurations and propagation scenarios. A LTERelease-8 pure precoding matrix indication (PMI) report based onfrequency division duplexing (FDD) DL channel sounding scheme may notprovide adequate information. This may be due to the increaseddifficulty in designing a codebook for higher-order MIMO. Furthermore,more PMI feedback overhead may be needed to fully exploit the benefitsof higher-order MIMO and MU-MIMO.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by embodiments of a systemand method for downlink (DL) channel sounding in wireless communicationssystems.

According to an embodiment, a user equipment (UE) configured tocommunicate wirelessly in a wireless network is provided. The UEincludes a processor and a non-transitory computer readable storagemedium storing programming for execution by the processor. Theprogramming includes instructions to receive multiple reference signalstransmitted over different beam directions from a base station, andreport channel state information to the BS based on channel measurementsof the multiple reference signals. The channel state informationcomprises at least one index of at least one of the multiple referencesignals. The at least one index is used to indicate at least one beamdirection over which the at least one of the multiple reference signalsare transmitted. The programming further includes instructions toreceive, from the BS, at least one data signal transmitted over the atleast one beam direction indicated by the at least one index. In oneexample, at least one channel measurement value of the at least one ofthe multiple reference signals satisfies a channel measurementrequirement for reporting. In the same example, or in another example,the channel state information further comprises at least one channelmeasurement value of the at least one of the multiple reference signals.In any one of the preceding examples, or in another example, the atleast one index is related to resource information of at least oneresource configured to transmit the at least one of the multiplereference signals. In any one of the preceding examples, or in anotherexample, at least one index comprises one index of a reference signalhaving a highest channel measurement value or two indexes of referencesignals having the highest channel measurement value and a lowestchannel measurement value.

In another embodiment, a method of operating a user equipment (UE)configured to communicate wirelessly in a wireless network. In thisembodiment, the method includes receiving multiple reference signalstransmitted over different beam directions from a base station (BS), andreporting channel state information to the BS, based on channelmeasurement of the multiple reference signals. The channel stateinformation comprises at least one index of at least one of the multiplereference signals. The at least one index is used to indicate at leastone beam direction over which the at least one of the multiple referencesignals are transmitted. The method further includes receiving at leastone data signal transmitted over the at least one beam directionindicated by the at least one index from the BS. In one example, atleast one channel measurement value of the at least one of the multiplereference signals satisfies a channel measurement requirement forreporting. In the same example, or in another example, the channel stateinformation further comprises at least one channel measurement value ofthe at least one of the multiple reference signals. In any one of thepreceding examples, or in another example, the at least one index isrelated to resource information of at least one resource configured totransmit the at least one of the multiple reference signals. In any oneof the preceding examples, or in another example, at least one indexcomprises one index of a reference signal having a highest channelmeasurement value or two indexes of reference signals having the highestchannel measurement value and a lowest channel measurement value. Acomputer program product and a non-transitory computer-readable mediumstoring a program for performing this method are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a communications system;

FIG. 2 a is a flow diagram of eNB operations in DL transparent channelsounding;

FIG. 2 b is a flow diagram of UE operations in DL transparent channelsounding;

FIG. 3 a is a diagram of iterative PCSRS based DL channel sounding inadvanced MIMO;

FIG. 3 b is a diagram of iterative PCSRS based DL channel sounding inCoMP transmission;

FIG. 4 a is a flow diagram of eNB operations in PCSRS based DL channelsounding in CoMP transmission;

FIG. 4 b is a flow diagram of UE operations in PCSRS based DL channelsounding in CoMP transmission;

FIG. 5 a is a flow diagram of eNB operations in PCSRS based differentialcodebook feedback;

FIG. 5 b is a flow diagram of UE operations in PCSRS based differentialcodebook feedback;

FIG. 6 is a diagram of PCSRS based channel sounding for CS/CB;

FIG. 7 a is a flow diagram of eNB operations in PCSRS based DL channelsounding for CS/CB; and

FIG. 7 b is a flow diagram of UE operations in PCSRS based DL channelsounding for CS/CB;

FIG. 8 illustrates a block diagram of an embodiment base station;

FIG. 9 illustrates a block diagram of an embodiment relay node; and

FIG. 10 illustrates a block diagram of an embodiment user device.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments are discussed in detail below.It should be appreciated, however, that the present application providesmany applicable inventive concepts that can be embodied in a widevariety of specific contexts. The specific embodiments discussed aremerely illustrative of specific ways to make and use the application,and do not limit the scope of the application.

Traditionally, CoMP transmission is only considered for access link (awireless link between an eNB (or relay node (RN)) and a UE) operation toimprove the coverage of the UE. However, CoMP transmission may also beapplied to backhaul link (a wireless link between an eNB and a RN)operation to enhance the coverage of the RN.

In a communications system with RNs, layered CoMP transmissions may beapplied to backhaul links as well as access links. With a backhaul CoMPtransmission, a RN may receive/transmit data from/to more than one eNBat a time, as long as data is available in more than one eNB to allowfor joint transmissions or coordinated transmission to a certain RN toenable inexpensive RN deployment. Backhaul CoMP transmissions may alsobe a way to control inter-cell interference. With access link CoMPtransmission, a UE can receive/transmit data from/to more than one RNsor eNBs, with the possibility of multiple access link CoMP transmissionsfor multi-hop RNs.

Both backhaul CoMP transmissions and access link CoMP transmissions maybe configured separately or jointly. When configured jointly, this maybe referred to as concatenated CoMP transmission. With separateconfiguration, it may be possible to utilize different CoMP transmissiontechnologies as well as feedback schemes in each CoMP transmissionlayer. A RN may act as both a CoMP transmission transmit point and/or aCoMP transmission receive point.

FIG. 1 illustrates a communications system 100. Communications system100 includes a number of eNBs, such as eNB 105 and eNB 106, that may beconnected to a gateway (G/W) 107 over a wired backhaul. The eNBs mayserve a number of UEs, such as UE 110, UE 111, and UE 112. Transmissionsfrom the eNBs to the UEs may occur directly, such as from eNB 105 to UE100, or indirectly, such as through a relay node (RN), including RN 115,RN 116, RN 117, and RN 118. For example, an indirect transmission mayoccur through a single RN, such as a transmission from eNB 105 to UE 111through RN 115, or through more than one RNs, such as a transmissionfrom eNB 106 to UE 112 through RN 116 and RN 118.

A RN may be used to relay transmissions made by an eNB to a UE or a UEto an eNB. The use of the RN may increase the coverage area of the eNB.As discussed previously, CoMP transmission has been proposed to increasethe coverage of a UE by allowing transmissions from multiple eNBs to bemade simultaneously to the UE, commonly referred to as an access link.However, CoMP transmission may also be used to increase the coverage ofa RN by allowing transmissions from multiple eNBs or RNs to be madesimultaneously to a the RN, commonly referred to as a backhaul link.

As shown in FIG. 1 , a first backhaul CoMP transmission from eNB 105 andeNB 106 may be made to RN 115 (shown as backhaul CoMP hop-1 125) and asecond backhaul CoMP transmission from RN 115 and RN 116 may be made toRN 117 (shown as backhaul CoMP hop-2 126). While an access link CoMPtransmission from RN 117 and RN 118 may be made to UE 112 (shown asaccess CoMP 130).

In LTE-A, the use of UE specific demodulation reference signal makestransparent DL transmission possible. With the UE specific demodulationreference signal, an eNB does not need to signal the transmissionmode/processing since the same transmission mode/processing is appliedto both the data and the UE specific demodulation reference signal. Thismay allow the eNB greater freedom in the selection of the transmissionmode, i.e., more scheduling flexibility.

However, in order for the eNB to determine the transmission mode, enoughchannel information should be available to the eNB in order for the eNBto make a decision. With explicit channel (H) feedback, the eNB obtainsthe entirety of the channel information. However, the feedback overheadis very high, especially for higher-order MIMO, MU-MIMO, and CoMPtransmission. With implicit channel feedback, such as PMI feedback, theeNB has to rely on the recommendation of the UE(s), which may restrictsome of the scheduling available at the eNB.

DL precoded common sounding reference signal (PCSRS) based channelsounding can be applied to CoMP channel sounding. DL PCSRS is eNBoriented, with the eNB configuring the precoding matrix adaptivelyaccording to deployment environment, such as antenna configuration,neighboring eNBs available for CoMP transmission, propagation scenario,UE distribution, and so forth. The sounding reference signal occupies DLchannel resources, therefore, less UL channel resources are required.Transparent DL channel sounding allows for the true transparentnon-codebook based precoding on the UE side with simple UEimplementation that does not require codebook searches or feedback.

FIG. 2 a illustrates a flow diagram of eNB operations 200 in DLtransparent channel sounding. eNB operations 200 may be indicative ofoperations taking place in an eNB, such as eNB 105, performing DLtransparent channel sounding to obtain feedback information to scheduleCoMP transmissions.

eNB operations 200 may begin with the eNB transmitting a common soundingreference signal that has been precoded with different processing (block205). The eNB may transmit the precoded common sounding referencesignals (PCSRS) periodically. The precoding may be based on certainpredefined matrices selected by the eNB with the predefined matricesbeing environment or antenna configuration dependent. Alternatively, theprecoding may be based on an initial full channel feedback provided byUE(s).

The resource elements over which the PCSRS are transmitted may belocated at predefined locations, to simplify UE detection. For example,the PCSRS corresponding to different precoding matrices may betransmitted cyclically according to a predefined pattern in time andfrequency. They may be carried by certain LTE-A physical resource block(PRB), for example, and information regarding the location of the PRB aswell as the cyclic pattern of the PCSRS may be broadcasted to all UEs.The eNB may also precode the PCSRS with a non-CoMP transmissionprecoding matrix (from a single cell) or a joint CoMP transmissionprecoding matrix (from multiple cells) to serve cell edge UEs or enableMU-MIMO transmission. The precoding is transparent to the cell edge UEs.

The eNB may then receive CQI(s) from each UE (block 210). The UEsmeasure the channel based on the PCSRS and reports the highest CQI or aspecified number of the highest CQIs corresponding to certain precodingprocessing. The UEs do not need to know the precoding used in the PCSRS.The UEs link the CQI with the corresponding precoding by including aPCSRS index in its report of the CQI. For example, the UEs may reportthe location of the PRB used to receive the PCSRS or the cyclic patternof the PCSRS.

The eNB may determine the best precoding matrix for each UE based on thereported CQI(s) (block 215). From the reported CQI(s) and the PCSRSindices from the different UEs, the eNB may be able to determine thebest precoding matrix for each UE. For example, the eNB finds the bestprecoding index for a UE based on a one-to-one relationship between theprecoding matrix index and the CQI index. The CQI information may alsobe used by the eNB for the selection of a modulation and coding scheme(MCS) if the corresponding precoding matrix is applied. eNB operations200 may then terminate. The information about the best precoding matrixand the corresponding CQI value may also be used by eNB to scheduleMU-MIMO transmission.

FIG. 2 b illustrates a flow diagram of UE operations 250 in DLtransparent channel sounding. UE operations 250 may be indicative ofoperations taking place in a UE, such as UE 110, participating in DLtransparent channel sounding to provide feedback information to an eNB,such as eNB 105, so that the eNB can schedule CoMP transmissions.

UE operations 250 may begin with the UE measuring a downlink channelusing the PCSRS transmitted by the eNB (block 255) and transmits thehighest CQI or a specified number of the highest CQIs to the eNB (block260). The UE knows where to make the measurements using the location ofthe PRBs or cyclic patterns broadcast by the eNB. The UE does not needto know the precoding used in the PCSRS and links the CQI (themeasurement) with a corresponding precoding by including a PCSRS indexin its report to the eNB. UE operations 250 may then terminate.

In order to optimize closed-loop (CL) performance for CoMP transmissionand advanced MIMO, sufficient spatial granularity may be required. ForUEs with medium to high mobility or for a situation with high correlatedantennas, the spatial resolution requirement may be relatively low sinceit is very difficult to track a narrow beam direction. However, rough(or coarse) beamforming may still provide a measure of CL gain whencompared with open-loop (OL) transmission. This may be especially truefor CoMP transmission when compared with OL CoMP transmission. Lowerspatial resolution means fewer hypotheses and lower PCSRS overhead.

For low mobility UEs, a higher spatial resolution may be required sinceit is possible to fine tune the beamformed beam. As an inverse to lowerspatial resolution, higher spatial resolution means more hypotheses andhigher sounding DL reference sequences. An efficient DL soundingapproach that enables a fast CL beamforming adaptation with reasonableDL sounding overhead is needed. Two possible solutions exist: iterativesounding and differential PMI feedback.

FIG. 3 a illustrates a diagram 300 of iterative PCSRS based DL channelsounding in advanced MIMO. Diagram 300 illustrates a portion of acommunications system comprising an eNB 305 and a number of UEs, such asUE 310, UE 315, and UE 316. UE 310 may be a UE with high mobility, whileUE 315 and UE 316 may be UEs with low mobility.

Iterative PCSRS may be applied to speed up the polling procedure for lowmobility UEs during sounding. Iterative PCSRS consists of rough tuningand fine tuning to reduce an overall number of hypotheses. Fine tuningmay be performed about a beam direction found in rough tuning. Thisdecreases PCSRS overhead and allows for fast CL adaptation.

eNB 305 transmits a number of rough tuning PCSRS precoded withprocessing matrices which separates the PCSRS roughly equally in space(shown as solid ovals 325 and 330). The UEs measure the rough tuningPCSRS and report back CQI(s), from which eNB 305 determines that a roughtuning PCSRS corresponding to oval 325 is reported as highest CQI for UE310. Similarly, for UEs 315 and 316, a rough tuning PCSRS correspondingto oval 330 is reported as highest CQI. Since UE 310 is a high mobilityUE, eNB 305 may not attempt to fine tune to increase spatial resolution.However, UEs 315 and 316 are low mobility UEs, therefore, eNB 305 mayincrease spatial resolution through fine tuning.

eNB 305 may achieve fine tuning by transmitting fine tuning PCSRSprecoding with processing matrices which separates the fine tuning PCSRSabout equally in space within a region encompassed by rough tuning PCSRScorresponding to oval 330 (shown as dotted ovals 335-338). The UEs (UEs315 and 316) again measure the fine tuning PCSRS and report back theCQI(s), from which eNB 305 determines the fine tuning PCSRScorresponding to the reported CQI(s) from the UEs.

Although shown to be a two-step process (rough tuning followed by finetuning), the fine tuning may be performed in several steps, with eachstep obtaining greater and greater spatial resolution. In practice, thenumber of fine tuning steps may be limited by factors such as themobility of the UEs, the amount of time (and other resources) that canbe dedicated to the fine tuning, and so forth.

FIG. 3 b illustrates a diagram 350 of iterative PCSRS based DL channelsounding in CoMP transmission. As shown in FIG. 3 b , diagram 350 issimilar to diagram 300 and the iterative PCSRS based DL channel soundingin CoMP transmission is substantially similar to the iterative PCSRSbased DL channel sounding in advanced MIMO. A difference being that morethan one eNB (eNBs 355 and 356 in FIG. 3 b ) are used to transmit thePCSRS.

FIG. 4 a illustrates a flow diagram of eNB operations 400 in PCSRS basedDL channel sounding in CoMP transmission. eNB operations 400 may beindicative of operations taking place in an eNB, such as eNB 105,performing PCSRS based DL channel sounding to obtain feedbackinformation to schedule CoMP transmissions.

eNB operations 400 may begin with the eNB transmitting rough tuningPCSRS that are precoded with processing matrices that separate the roughtuning PCSRS roughly equally space, rough beams allow for identificationwith less spatial granularity (block 405). The eNB may then receive CQIreports from the UEs, wherein the UEs have performed channelmeasurements of the rough tuning PCSRS and selected the strongest CQI(or a specified number of the strongest CQI) and reported them back tothe eNB (block 407). The reported CQI(s) correspond to a particularprecoding matrix and corresponding PCSRS index.

From the reported CQI, the eNB may determine the best PCSRS (and hencethe best beam direction) for each UE (block 409). The eNB may use thebeam direction as a baseline for CL precoding processing (block 411).The beam direction may also be used as a fall back for precodingprocessing when the eNB needs to override the recommendations of theUEs, for example.

The eNB may then transmit fine tuning PCSRS that have been precoded withprocessing matrices that covers a region covered by the rough tuningPCSRS selected by the UEs (block 413). If there are more than one roughtuning PCSRS to fine tune, each of the additional rough tuning PCSRS maybe fine tuned one at a time with additional fine tuning PCSRS. The eNBmay then receive CQI reports from the UEs, wherein the UEs haveperformed channel measurements of the fine tuning PCSRS and selected thestrongest CQI (or a specified number of the strongest CQI) and reportedthem back to the eNB (block 415). The fine tuning PCSRS are transmittedto the low mobility UEs. The eNB may identify the low mobility UEs thatneed additional fine tuning and informs the UEs or the UE in CL modeknows that it is in need of fine tuning. The reported CQI(s) correspondto a particular precoding matrix and corresponding PCSRS index. From thereported CQI, the eNB may determine the best fine tuning PCSRS (andhence the best beam direction) for each UE (block 417) and eNBoperations 400 may terminate.

As discussed previously, if there are multiple rough tuning PCSRS tofine tune, then the eNB may repeat the transmission of fine tuning PCSRSfor each of the rough tuning PCSRS (with the fine tuning PCSRS beingspecifically designed for each of the rough tuning PCSRS). Furthermore,the fine tuning step may be performed multiple times to obtain aprogressively finer and finer spatial resolution.

FIG. 4 b illustrates a flow diagram of UE operations 450 in PCSRS basedDL channel sounding in CoMP transmission. UE operations 450 may beindicative of operations taking place in a UE, such as UE 100,participating in PCSRS based DL channel sounding to provide feedbackinformation to an eNB, such as eNB 105, so that the eNB can scheduleCoMP transmissions.

UE operations 450 may begin with the UE measuring a downlink channelusing the coarse tuning PCSRS transmitted by the eNB (block 455). The UEmay then report the highest measured CQI or a specified number ofhighest CQI (block 457). UE operations 450 may continue with the UEmeasuring the downlink channel using the fine tuning PCSRS transmittedby the eNB (block 459). The UE may then report the highest measured CQIor a specified number of highest CQI (block 461) and UE operations 450may terminate.

If there are several PCSRS that have the same highest measured CQI, thenthe UE may report all of the indices. Alternatively, the UE may select aspecified number of the indices to report. Alternatively, the UE mayselect one index to report. The selection of the index (or indices) maybe performed at random, based on eNB operating conditions (load, numberof UEs served, UE priority, and so forth).

As discussed previously, not all UEs will participate in the measurementof the downlink channel with the fine tuning PCSRS. The UEs may receiveindications from the eNB indicating that they are to participate in thefine tune step. Alternatively, the UEs may be operating in CL mode andknow that they are to participate in the fine tune step.

Differential PMI feedback combines PMI feedback with PCSRS DL channelsounding. A differential codebook may be applied to reduce the PMIfeedback overhead since a smaller codebook may be used. Furthermore, thedifferential codebook may be used to enhance the spatial resolution of abase codebook with the same codebook size, which reduces the codebooksearch space. Additionally, the differential codebook may be used totrace the change of the channel as a reference for the differentialcodebook search being a precoded demodulation reference sequence carriedby a data channel.

FIG. 5 a illustrates a flow diagram of eNB operations 500 in PCSRS baseddifferential codebook feedback. eNB operations 500 may be indicative ofoperations taking place in an eNB, such as eNB 105, performing PCSRSbased differential codebook feedback to obtain feedback information toschedule CoMP transmissions.

eNB operations 500 may begin with the eNB initiating an initial channelsounding by sending PCSRS precoded with eNB selected precoding matrices(block 505). According to a preferred embodiment, the precoding matricesare selected with large granularity. The eNB may then receive indices ofPCSRS having highest measured CQI as well as the CQI value itself fromthe UEs (block 507).

In addition to the PCSRS indices and CQI, the eNB also receives a PMIfrom a differential codebook search performed by the UEs (block 509).The eNB then determines a precoding matrix based on the reported PMI andthe received PCSRS index and CQI (block 511). The eNB may use thereported PCSRS from the UEs to verify the PMI feedback and eNBoperations 500 may terminate.

The eNB may be able to override the PMI recommendation from the UEs(block 509) with a precoding matrix that it computes on its own from thePCSRS indices and CQI received from the UEs.

FIG. 5 b illustrates a flow diagram of UE operations 550 in PCSRS baseddifferential codebook feedback. UE operations 550 may be indicative ofoperations taking place in a UE, such as UE 110, participating in PCSRSbased differential codebook feedback to provide feedback information toan eNB, such as eNB 105, so that the eNB can schedule CoMPtransmissions.

UE operations 550 may begin with the UE measuring a DL channel using thePCSRS transmitted by the eNB, wherein the PCSRS have been precoded withprecoding matrices, wherein the precoding matrices have largegranularity (block 555) The UE may report an index of a PCSRScorresponding to a highest measured CQI to the eNB (block 557). Inaddition to the index of the PCSRS, the UE may also report the highestmeasured CQI value.

The UE may also narrow down the codebook search by performing adifferential codebook search along a direction of the PCSRS having thehighest measured CQI (block 559). The PCSRS that resulted in the highestmeasured CQI may be used by the UE as a reference in the differentialcodebook search. In some scenarios, only UEs participating in CoMPtransmissions need to perform the differential codebook search, whereinthe UEs are based on DL signaling or a present CQI threshold. In otherscenarios, UEs not participating in CoMP transmission may also need toperform the differential codebook search to provide more preciseprecoding information for MU-MIMO transmission. The UE reports to theeNB a best PMI from the differential codebook search (block 561) and UEoperations 550 may then terminate.

If there are several PCSRS that have the same highest measured CQI, thenthe UE may report all of the indices. Alternatively, the UE may select aspecified number of the indices to report. Alternatively, the UE mayselect one index to report. The selection of the index (or indices) maybe performed at random, based on eNB operating conditions (load, numberof UEs served, UE priority, and so forth).

FIG. 6 illustrates a diagram 600 of PCSRS based channel sounding forCS/CB. Diagram 600 illustrates a portion of a communications systemcomprising a first eNB 605 and a second eNB 610. First eNB 605 may beserving UE 615 and UE 616, while second eNB 610 serves UE 616. First eNB605 and second eNB 610 may be using CoMP transmission to serve UE 616.

Based on measurements of PCSRS precoded with different precodingmatrices transmitted by first eNB 605 and second eNB 610, UE 616 mayreport back to both eNBs PCSRS indices corresponding to highest measuredCQI or both highest and lowest measured CQI. In other words, UE 616reports back to the eNBs the strongest and weakest beam directions. Forexample, dashed oval 620 may represent a weakest beam direction forfirst eNB 605 and solid oval 625 may represent a strongest beamdirection for second eNB 610. Using the information provided by the UEs,the eNBs can schedule transmissions to its own UEs. For example, knowingthe strongest beam direction for first eNB 605, second eNB 610 mayschedule transmissions to its own UEs in the same beam direction at thesame time that first eNB 605 is transmitting to its UEs in the strongestbeam direction.

FIG. 7 a illustrates a flow diagram of eNB operations 700 in PCSRS basedDL channel sounding for CS/CB. eNB operations 700 may be indicative ofoperations taking place in an eNB, such as eNB 105, performing PCSRSbased DL channel sounding for CS/CB.

eNB operations 700 may begin with the eNB transmitting PCSRS precodedwith different precoding matrices (block 705). The eNB may then receivean index of PCSRS corresponding to PCSRS having highest measured CQI orindices of PCSRS having highest and lowest measured CQI (block 707). TheeNB may inform its neighboring cells (eNBs) of weakest beam directionsof its cell edge UEs (block 709). The eNB may schedule transmission toUEs in same beam direction as weakest beam direction of neighboringcells (block 711) and eNB operations 700 may then terminate.

FIG. 7 b illustrates a flow diagram of UE operations 750 in PCSRS basedDL channel sounding for CS/CB. UE operations 750 may be indicative ofoperations taking place in a UE, such as UE 100, participating in PCSRSbased DL channel sounding for CS/CB.

UE operations 750 may begin with the UE measuring a DL channel using thePCSRS transmitted by the eNB (block 755). The UE may report either anindex of a PCSRS having highest measured CQI or indices of PCSRS havinghighest measured CQI and lowest measured CQI (block 757) and UEoperations 750 may then terminate.

If there are several PCSRS that have the same highest (or lowest)measured CQI, then the UE may report all of the indices. Alternatively,the UE may select a specified number of the indices to report.Alternatively, the UE may select one index to report. The selection ofthe index (or indices) may be performed at random, based on eNBoperating conditions (load, number of UEs served, UE priority, and soforth).

There may be different CoMP transmission feedback schemes for backhaulCoMP transmission and access link CoMP transmission. For example, anexplicit feedback scheme may be used for backhaul CoMP transmission.With backhaul CoMP transmissions fixed RNs, the explicit feedback schememay have an acceptable level of UL feedback overhead.

A hybrid DL channel sounding scheme may be used for access link CoMPtransmission. The hybrid DL channel sounding scheme may include bothexplicit and implicit DL channel sounding. The explicit DL channelsounding may be performed as an initial DL channel sounding, with theUEs feeding back information regarding the DL channel to the servingeNB. For CoMP transmission, the UE feeds back the DL channel to theneighboring eNBs as well. The implicit channel sounding may be used tokeep track of changes in the DL channel. Additionally, an explicitfeedback scheme may be used for fixed UEs, while a PCSRS based feedbackscheme may be used for UEs that are mobile (low/medium/high mobility).The PCSRS based feedback scheme may use non-adaptive PCSRS basedsounding for medium and high mobility UEs and adaptive PCSRS basedsounding and PCSRS based differential codebook feedback for low mobilityUEs.

Furthermore, hybrid DL channel sounding schemes may be used forhigher-order SU-MIMO or MU-MIMO (referred to collectively ashigher-order MIMO). In higher-order MIMO systems, a non-adaptivetechnique, such as non-adaptive PCSRS based sounding, may be used formedium and high mobility UEs or for a situation with highly correlatedantennas, while adaptive PCSRS based sounding may be used for lowmobility UEs. In high-order MIMO systems with uncorrelated transmitantennas, a PCSRS based differential codebook feedback technique may beused.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the application as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture,composition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present application, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present application. Accordingly, the appended claims are intendedto include within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A method of operating a base station configuredto communicate wirelessly in a wireless network, the method comprising:transmitting, by the base station to a user device, precoded referencesignals over different beam directions; receiving, by the base stationfrom the user device, an index that explicitly identifies one of theprecoded reference signals transmitted to the user device; andtransmitting, by the base station to the user device, a beam-formed datasignal in accordance with the index received from the user device. 2.The method of claim 1, wherein the beam-formed data signal istransmitted from a single base station.
 3. The method of claim 1,wherein the beam-formed data signal is a joint transmission coordinatedbetween multiple base stations.
 4. The method of claim 3, furthercomprising: sharing a first weakest beam direction between two of themultiple base stations; and scheduling transmissions to the user deviceusing precoding matrices corresponding to the first weakest beamdirection.
 5. The method of claim 3, wherein control signaling forcoordinating the joint transmission is communicated between at least twoof the multiple base stations via an X2 link.
 6. The method of claim 1,wherein the beam-formed data signal is a joint transmission coordinatedbetween the base station and at least one relay node on a backhaul linkand on an access link.
 7. The method of claim 1, wherein the wirelessnetwork is a long term evolution (LTE) network, the base stationcomprises an enhanced Node B (eNB), and the user device comprises userequipment (UE).
 8. The method of claim 1, wherein the transmitting thebeam-formed data signal to the user device comprises computing aprecoding matrix for the user device.
 9. The method of claim 1, whereinthe transmitting the beam-formed data signal to the user devicecomprises: performing a coarse beam adjustment based on the indexreceived from the user device; and performing a fine beam adjustmentafter performing the coarse beam adjustment.
 10. The method of claim 9,wherein performing the fine beam adjustment comprises: receiving, fromthe user device, precoding matrix indication (PMI) differential codebookfeedback, and adjusting a beam direction in accordance with the PMIdifferential codebook feedback.
 11. The method of claim 1, furthercomprising: performing a coarse tuning with the user device to achieve afirst degree of spatial granularity; and performing a fine tuning withthe user device to achieve a second degree of spatial granularity. 12.The method of claim ii, wherein performing the coarse tuning comprises:transmitting, to the user device, precoded reference sequences, whereinthe precoded reference sequences are precoded with precoding matricesseparating the precoded reference sequences; receiving, from the userdevice, channel information related to the; and computing a best roughtune precoding matrix for the user device based on the channelinformation.
 13. The method of claim 12, wherein the channel informationcomprises channel quality measurements for each of the precodedreference sequences.
 14. The method of claim 12, wherein the channelinformation comprises a highest channel quality measurement for aprecoded reference sequence out of the precoded reference sequences. 15.The method of claim 14, further comprising generating one or moreadditional precoded reference sequences based on the best rough tuneprecoding matrix, wherein the best rough tune precoding matrixcorresponds to the precoded reference sequence having the highestchannel quality measurement.
 16. A base station in a wireless network,the base station comprising: a processor; and a non-transitory computerreadable storage medium storing programming for execution by theprocessor, the programming including instructions to: transmit, to auser device, precoded reference signals over different beam directions;receive, from the user device, an index that explicitly identifies oneof the precoded reference signals transmitted to the user device; andtransmit, from the user device, a beam-formed data signal in accordancewith the index received from the user device.
 17. The base station ofclaim 16, wherein the beam-formed data signal is transmitted from asingle base station.
 18. The base station of claim 16, wherein thebeam-formed data signal is a joint transmission coordinated betweenmultiple base stations.
 19. The base station of claim 18, wherein theprograming further includes instructions to: share a first weakest beamdirection between two of the multiple base stations; and scheduletransmissions to the user device using precoding matrices correspondingto the first weakest beam direction.
 20. The base station of claim 18,wherein control signaling for coordinating the joint transmission iscommunicated between at least two of the multiple base stations via anX2 link.
 21. A method of operating a user device configured tocommunicate wirelessly in a wireless network, the method comprising:receiving, by the user device from a base station, precoded referencesignals; transmitting, by the user device to the base station, an indexexplicitly identifying one of the precoded reference signals receivedfrom the base station; and receiving, by the user device from the basestation, a beam-formed data signal.
 22. The method of claim 21, whereinthe beam-formed data signal is received from a single base station. 23.The method of claim 21, wherein the beam-formed data signal is a jointtransmission coordinated between multiple base stations.
 24. The methodof claim 23, wherein control signaling for coordinating the jointtransmission is communicated between at least two of the multiple basestations via an X2 link.
 25. The method of claim 21, wherein thebeam-formed data signal is a joint transmission coordinated between thebase station and at least one relay node on a backhaul link and on anaccess link.
 26. The method of claim 21, wherein the wireless network isa long term evolution (LTE) network, the base station comprises anenhanced Node B (eNB), and the user device comprises user equipment(UE).
 27. The method of claim 21, wherein further comprising: receiving,from the base station, precoded reference sequences, wherein theprecoded reference sequences are precoded with precoding matricesseparating the precoded reference sequences; estimating channelinformation based on the precoded reference sequences; and transmitting,to the base station, the channel information.
 28. The method of claim27, wherein the channel information comprises channel qualitymeasurements for each of the precoded reference sequences.
 29. Themethod of claim 27, wherein the channel information comprises a highestchannel quality measurement for a precoded reference sequence out of theprecoded reference sequences.
 30. A user device in a wireless network,the user device comprising: a processor; and a non-transitory computerreadable storage medium storing programming for execution by theprocessor, the programming including instructions to: receive, from abase station, precoded reference signals; transmit, to the base station,an index that explicitly identifies one of the precoded referencesignals received from the base station; and receive, from the basestation, a beam-formed data signal.
 31. The user device of claim 30,wherein the beam-formed data signal is received from a single basestation.
 32. The user device of claim 30, wherein the beam-formed datasignal is a joint transmission coordinated between multiple basestations.
 33. The user device of claim 32, wherein control signaling forcoordinating the joint transmission is communicated between at least twoof the multiple base stations via an X2 link.
 34. The user device ofclaim 30, wherein the beam-formed data signal is a joint transmissioncoordinated between the base station and at least one relay node on abackhaul link and on an access link.
 35. The user device of claim 30,wherein the wireless network is a long term evolution (LTE) network, thebase station comprises an enhanced Node B (eNB), and the user devicecomprises user equipment (UE).
 36. The user device of claim 30, whereinthe programming further includes instructions to: receive, from the basestation, precoded reference sequences, wherein the precoded referencesequences are precoded with precoding matrices separating the precodedreference sequences; estimate channel information based on the precodedreference sequences; and transmit, to the base station, the channelinformation.
 37. The user device of claim 36, wherein the channelinformation comprises channel quality measurements for each of theprecoded reference sequences.